In general, a differential capacitive sensor is a surface micromachine device typically having three electrically isolated layers of polysilicon wherein at least one application for a differential capacitive sensor is accelerometers. The first layer of the sensor is typically rigidly attached to the substrate and is electrically isolated from the substrate by an oxide layer. The first layer forms the bottom plate of a three layer capacitor. The second layer of the sensor is sandwiched between the first layer and a third layer and is typically supported by a set of beams running down to the first layer. The second layer forms the middle plate. The third layer is above the first two layers and is supported in such a manner as to remain rigid. The third layer forms the top plate of the three layer capacitor. The middle plate is free to move in response to an applied force. Thus, an accelerating force produces a perceptible change in the capacitance between the first and second plates and the second and third plates. Further, an electrostatic force, for example, one generated by a voltage source, can also cause the middle plate to move.
If the middle plate moves towards the bottom plate, the capacitance between the middle plate and the bottom plate increases (because the distance has decreased), while the capacitance between the top plate and the middle plate correspondingly decreases. By maintaining the middle plate at a central position with a balanced amount of top and bottom electrostatic forces, external accelerating forces acting on the middle plate can be measured.
Circuits that sense a capacitive change due to motion of the middle plate and exert no appreciable electrostatic force to bring back the middle plate to a central position rely on the mechanical elasticity of the beams supporting the middle plate to provide a means of balancing external accelerating forces. These sensor circuits are sensitive to manufacture variability from sensor to sensor.
Further, circuits that sense motion but do not counteract the motion with an electrostatic force are referred to as open loop circuits. On the other hand, a closed loop circuit provides a means of monitoring the middle plate at a central position.
One example of a closed loop circuit for maintaining the middle plate at a central position is fully described in an IEEE Solid-State Sensor and Actuator Workshop article entitled "Wide Dynamic Range Direct Digital Accelerometer" by Widge Henrion et al. The accelerometer is made by bonding three wafers together. The middle plate includes two electrically isolated plates wherein a first plate is used for sensing the position of the middle plate and a second plate is used for applying an electrostatic force to the middle plate. However, this accelerometer requires numerous masking layers and is difficult to manufacture.
Hence, what is needed is a circuit for sensing and controlling the differential capacitance between the first and second plates and the second and third plates with an electrically common second plate.